496 CLARK AND BIBBINS 
of the Potomac group took place, by means of which the higher 
strata were gradually cut off to the southward. The details 
of this transgression will be further explained in the subsequent 
pages. 
The basal deposits of the Potomac group, which have been 
described under the name of the Patuxent formation, indicate 
in their arkosic character, their proximity to the ancient con- 
tinent, the rocks of which had suffered extensive disintegration. 
These features so pronounced where the deposits he adjoining 
geneissic or granitic rocks largely disappear where these rocks 
are poorly developed and where the deposits themselves were 
evidently laid down at some distance from the old shore line. 
Rapid deposition in shallow waters is seen in the cross-bedded 
character of the strata and their rapid change in character 
The appearance of clay balls, so widely disseminated at certain 
points, as above described, indicates the shallowing of the seas 
and the breaking down of pre-existing clay beds by wave action 
and the incorporation of the rolled materials by the later 
deposits. 
_ The close of the Patuxent epoch was evidently marked by a 
gradual elevation of the deposits and the trenching of the surface 
by streams, so that a series of broad, shallow valleys was formed 
While this was going forward, a landward depression of the con- 
tinent border evidently took place, producing series of long 
marshes in the ancient valleys, in which sedimentation was slow 
and in which swamp vegetation flourished. The tough clays, 
filled with lignitic accumulation, in which the tree trunks are 
sometimes found erect, with their roots still intact, can hardly 
find a satisfactory explanation upon any other basis. It was in 
these ancient marshes that the iron was deposited. It is also 
probable that some connection existed between these marshes 
and the area of basic eruptive rocks of central and northern 
Maryland. It was in these marshes that the bones of Dinosauria 
became entombed which, with the evidences of dense vegeta- 
tion, suggests a sub-tropical climate. 
The marked line of unconformity separating the Patapsco 
